The invention generally relates to generating a bias voltage.
Referring to FIG. 1, a differential amplifier 10 typically includes differential transistors 12 that receive and amplify a differential input signal (that appears across input terminals 16 and 18 of the amplifier 10) to produce a differential output signal across output terminals 26 and 28. The differential amplifier 10 may also include other transistors, such as a current mirror transistor 11 and cascode transistors 14.
The differential amplifier 10 may be a low noise, radio frequency (RF) amplifier, which means the transistors of the amplifier 10 should be designed to be relatively fast and contribute a relatively small level of noise to the differential output signal. A potential challenge in using such a transistor is that the transistor typically can sustain only a relatively small (1.2 volts, for example) voltage across any two of its terminals while the supply voltage may be much higher (3.3 volts, for example).
Bias circuitry 20 provides bias voltages to set the various operating points of the transistors of the amplifier 10. For example, as depicted in FIG. 1, the differential transistors 12 may be biased by a reference voltage (called “VBD”) that is coupled to the gate terminals of the transistors 12, and the cascode transistors 14 may be biased by a reference voltage (called “VBC”) that is coupled to the gate terminals of the transistors 14.
As depicted in FIG. 1, both the VBD and VBC reference voltages may be generated by ground-referenced voltage reference circuits, such as the depicted circuits 24 and 22, respectively. “Ground-referenced” means that each of the reference circuits 22 and 24 maintains its output reference voltage with respect to ground.
Because the voltage reference circuits 22 and 24 are referenced to ground, the VBC and VBD reference voltages do not vary with a supply voltage (called “VDD”) that is coupled to the differential amplifier 10. This presents challenges because the operating points of the amplifier's transistors also depend on the voltage differences between the VDD supply voltage and the VBD and VBC reference voltages.
More specifically, the VDD supply voltage typically is directly supplied by or is a function of the external supply voltage that is furnished to an integrated circuit package that contains the differential amplifier 12; and as a result, the VDD supply voltage is expected to be not at a specific voltage level, but rather the expected voltage level of the VDD supply voltage is defined by a range, such as 2.7 to 3.3 volts. The actual voltage level of the VDD supply voltage depends on the external supply voltage. Although the expected value of the VDD supply voltage is defined by a range of voltages, the VBC and VBD reference voltages are designed to be specific voltages with respect to ground, which do not vary with the actual VDD supply voltage. Therefore, non-optimum operating points may be established in the differential amplifier 10, depending on the actual VDD supply voltage.
One way to ensure that the reference voltages vary with the VDD supply voltage is to use voltage reference circuits that are referenced to the VDD supply voltage instead of being referenced to ground. However, for a low noise amplifier design, this solution may be undesirable because the VDD supply voltage may be relatively noisy and thus, may introduce an undesirable level of noise into the differential amplifier 10. This may also be a problem because the input circuit (to the amplifier 10) may be ground-referenced.
Thus, there is a continuing need for a better technique and/or system to bias circuitry, such as a differential amplifier. There is also a continuing need for a bias circuit that furnishes the appropriate bias voltages as the supply voltage scales down.